In consideration of the fact that many foods and food accessories such as tea, coffee, cocoa, contain certain purin bodies, it is important that we should endeavour to trace them through their various manifestations in the body. One of the most important changes to which purin bodies are subjected is oxidation, and their highest product of oxidation is uric acid. This is a complex compound form from which two molecules of urea can easily be split off, and may be looked upon as a combination of urea and an acid containing three atoms of carbon. The acid best answering this description is acrylic acid, and it is interesting to know that from the chemical point of view it is a close relative of lactic acid - hydracrylic acid being an isomer of lactic acid - and that Horbaczewski has synthesised uric acid from tri-chlorlactic acid and urea, and also by heating urea and glycocoll together.

In reptiles and birds the greater part of the nitrogen leaves the body as uric acid, but this never obtains in mammals, where only a small proportion - about 3 per cent. - of the nitrogen ingested appears in the urine as uric acid, while quite 86 per cent. is excreted as urea. Probably the explanation of this difference may be found in its great insolubility, one gram of uric acid requiring at the body temperature seven litres of water for its solution and being only slightly soluble in cold water. For this reason it is discharged in birds and reptiles in the solid form of acid ammonium urate, and no disadvantage is associated with this method of excretion, as in both of those classes of animals the urine is expelled from the ureters into the cloaca by the pressure of the oncoming faeces. Were this to take place in mammals the microscopical uric acid crystals would give rise to excessively painful symptoms. Consequently, it is fortunate that in them this form of nitrogen is at a minimum in the urine, so that it may be possible to keep it in solution.

In man only .5-1 gram of uric acid is expelled in the twenty-four hours, whereas about 40 grams of urea are excreted in the same time.

The purin bodies are now well known to be derivatives of the nucleo-proteins. These latter are compounds of protein with a phosphorus-containing organic acid called nucleic acid, and are found both in the cytoplasm and the nuclei of the cells.

Nuclein, which is identical with the chromatin of the cell nuclei, contains quite 10 per cent. of phosphorus, and on disintegration forms protein and nucleic acid. The nucleo-protein of the cell protoplasm on the other hand contains only 1 per cent. of phosphorus.

The composition of nucleo-protein will probably be better grasped by the following diagrammatic representation: -

Two forms of nucleic acid have been differentiated.

(1) A pure form, yielding on decomposition -

(a) A carbohydrate of the hexose group,

(b) Phosphoric acid,

(c) Two purin bases - adenin and guanin,

(d) Three pyrimidin bases - cytosine, uracil, and thymine, the last yielding thyminic acid.

(2) Guanylic acid - a less complex form of nucleic acid, which can be decomposed into:

(a) A carbohydrate of the pentose group (1. xylose),

(b) Phosphoric acid,

(c) Guanin alone.

Uric acid, which is dibasic, never occurs pure in the body, but is always combined with bases as urates - normal sodium urate and acid sodium urate. It is the chief end-product of the katabolism of the cell nuclei, and Emil Fischer has shown that among the products of decomposition of their nuclein, the purin bodies are the most important. In the human body these are always in organic combination and never in the form of salts, but they possess a common nucleus, C5N4. Purin, C5H4N4 (or purine), is itself only of theoretical interest, as it has never been discovered in the body.

The other members of the series occurring in foods are: -

There are many other purin derivatives, but only a few are of importance as occurring in tea, coffee, cocoa. Theophyllin in tea, theobromin in cocoa are dimethyl oxypurins, and caffein in coffee is trimethyl oxypurin. They are, however, more usually classed together as methyl xanthins.